Advances in technologies associated with electrical circuitry have led to great improvements in many fields. For example, the miniaturization of transistors has enabled computational speeds and data storage capacities for computers considered impossible only a few years ago.
The field of nanotechnology, involving materials formed and utilized on a nanometer scale, has developed over the last several years as the next step in the ongoing attempt to further miniaturize materials. Some of the most exciting materials to be discovered in the field of nanotechnology are single-walled and multi-walled carbon nanotubes. Carbon nanotubes exhibit many desirable properties including high tensile strength, high surface area, light weight per unit length, and the capacity to conduct very high current densities, exceeding 107 A/cm2. In particular, the electron transport properties of carbon nanotubes make them excellent candidates for incorporation into nanoscale circuit devices.
Current theories suggest that three-terminal Y-junction nanotubes can exhibit the gating behavior characteristic of traditional transistors. As such, methods of forming Y-junction nanotubes have been devised. Presently known methods present many problems, however. For example, Li, et al. (U.S. Pat. No. 6,325,909) disclose a method including the formation of an alumina template defining branched channels, growth of nanotubes in the channels via pyrolysis of acetylene using cobalt catalysis, and destruction of the template with acid to recover the branched nanotubes. This is a very detailed and time-consuming process, however, that can take as long as a week to complete. In addition, there exists the danger that the product nanotubes can be damaged in the process of destroying the template.
According to another proposed method for forming Y-branched nanotubes, disclosed by Satishkumar, et al. (“Y-junction Carbon Nanotubes,” Applied Physics Letters, 77(16), 2530 (2000)), networks of multi-walled nanotubes with random and frequent branches are synthesized via pyrolysis of metallocene/thiophene vapors. The process affords little or no control over the branching mechanism, however, and the products appear to have a high ratio of nanotubes with branching stubs, rather than true Y-junctions that could be used in electrical applications.
One additional method, disclosed by Li, et al. (“Straight Carbon Nanotube Y Junctions,” Applied Physics Letters, 79(12), 1879 (2001)), describes a process including the pyrolysis of methane over MgO-supported cobalt catalysts. The catalysts are prepared by dissolving Co(NO3)2.6H2O in ethanol and then immersing MgO powder in the solution and sonicating. The mixture is then calcined for about 14 hours and reduced at 1000° C. for one hour under hydrogen and nitrogen. Replacement of the nitrogen with methane results in the formation of branching nanotubes with fixed angles between each branch. These networks of branched nanotubes also form a majority of the junctions with two long arms and a third arm that is no more than a branching stub, similar to the products of Satishkumar, et al.
What is needed in the art is a method for producing branched carbon nanotubes such as Y-junction carbon nanotubes that can be easily sized for bulk formation processes. In addition, what is needed in the art is a method for forming branched carbon nanotubes that includes a scheme to control the location and length of the branches formed along the length of the nanotubes.